As in adults, overweight and obesity are rising at alarming rates among children and adolescents. To counter this trend, increasing physical activity and decreasing sedentary behavior has become a research priority at the NIH. Successful interventions to increase physical activity as a "permanent lifestyle change" (PA-01- 017), however, cannot be developed without a better understanding of the determinants of physical activity behaviors. Our research and that of others among adults and adolescents has shown that how we perceive ourselves athletically and how others perceive us are plausible determinants of whether we are physically active. Long-term physical activity patterns that are sustained over many years likely reflect a core aspect of the self: a relevant self-definition as a physically active person that is initiated in childhood. The proposed project builds upon two funded studies among children and adolescents to provide important longitudinal data on the development of athletic self-concept and its relation to physical activity behavior. Using latent growth curve analysis, Aim 1 is to assess developmental differences in athletic self-concept in a multi-cohort, multi-occasion study of 7th and 8th grade adolescents (n=932) over 3 years, across the transition from junior high to senior high school. Aim 2 is to evaluate causal ordering or the longitudinal relations between athletic self-concept and physical activity over the transition: how are initial status and growth in these two constructs related over time? Aim 3 is to identify and evaluate risk factors and predictors of status and change over time in athletic self-concept and physical activity, with a focus on demographic, child weight, parental, and peer influences. These efforts will result in new insights into the complexity of self-development across childhood and adolescence, and will become the foundation for developing successful interventions that target physical activity behavior change. Our project represents an innovative approach to understanding and increasing physical activity in the primary prevention of cancer.